Method of applying a coating to a part

ABSTRACT

The invention comprises a method of applying a coating to a steel part in which a coating of aluminium powder and organic resin is electrophoretically applied to the part, the coating is heated to drive off the resin, and the aluminium coating is impregnated with a high temperature resistant base material.

ll'nited States Patent [19] Ballard [451 Jan. 22, 1974 METHOD OF APPLYING A COATING TO A PART [75] Inventor: Norman Edmund Ballard, Derby,

England [7 3] Assignee: Rolls-Royce (1971) Limited,

London, England [22] Filed: June 12, 1972 [21] Appl. No.: 261,814

[30] Foreign Application Priority Data June 17, 1971 Great Britain 28369/71 [52] US. Cl. 204/181 [51] Int. Cl C23!) 13/00 5 8] Field of Search .l. 204/181 [56] References Cited UNITED STATES PATENTS 3,630,869 12/1971 Mann 204/18] 3,676,308 7/1972 Brown 204/181 Primary Examiner-Howard s. Williams Attorney, Agent, or Firm-Cushman, Darby & Cushman [57] ABSTRACT 14 Claims, No Drawings METHOD OF APPLYING A COATING TO A PART This invention relates to a method of applying a coating to a steel part.

It is often desirable to apply coatings to steel components which will protect the components from high temperatures and from atmospheric corrosion. However, a number of paints and similar coatings which provide good results as a coating are often correspondingly difficult to apply to a part.

The present invention provides a method of applying a particular high temperature resistant coating to a steel part.

According to the present invention a method of applying a coating to a steel part comprises electrophoretically applying a coating of aluminum powder and organic resin to the part, heating the part to drive off the organic resin, and subsequently impregnating the coating with a high temperature resistant base material.

We prefer to use an acrylic resin as the organic resin, while the high temperature resistant base material is preferably potassium silicate. In this case it is preferable to stove the impregnated coating.

In a preferred embodiment the initial coating is performed from an aqueous solution. Thus the electrophoretic coating may be performed from a solution comprising one part of an acrylic resin, from 2 to 20 parts by weight offine powdered aluminum and water to make a workable consistency.

We prefer to use 1 to 2 parts of the resin, 2 to 16 parts of the aluminum and'l to 6 parts water.

Examples of the method were as follows:

EXAMPLE 1 100 grams of a 50 percent aqueous solution of acrylic resin known as Synocryl 84/S and obtained from Messrs. Cray Valley Products was mixed with 400 grams fine powdered grease free aluminum of particle size to'l0 microns (400 grade) and 250 grams of water. The mixture was ball milled for one hour to produce a homogenous mixture.

Test panels of 12 percent chromium steel were immersed in the mixture and an electric potential applied between the panels and a fixed electrode so that the panels were given negative polarity and consequently were coated with the aluminum and resin by electrophoresis. We found that a current density of 0.1 amps per square inch gave best results, and the coatingwas continued for 30 seconds to give a coating of between 0.4 and 0.8 thousandths of an inch thickness.

The test panels were then removed from the mixture and the coating was stoved for 30 minutes at 150C. The panels were then further heated to 560C; this caused the acrylic resin to be driven off and also diffused the aluminum coating. This temperature was maintained for two hours.

The panels were then provided with a porous aluminum coating, to complete the coating the panels were allowed to cool to 100C and an aqueous solution of potassium silicate comprising 1 part by volume of the silicate in 2 parts by volume of water was brushed onto the coating until it had completely filled the pores of the aluminum. The panels were then finally stoved for 30 minutes at 150C. The actual potassium silicate used was Crosfield Grade 66 having a mean molecular ratio of SiO :K O of 3.21 and a specific gravity of 1.33.

Test panels which had been thus coated were then subjected to corrosive conditions to evaluate the coating. Each panel was subjected to 10 cycles of a heating programme which involved two hours heating at 450C followed by 22 hours of neutral salt spray at room temperature. After this test the test panels were in good condition and the coating was almost undamaged as was the steel substrate. The panels were also tested by immersion in various fluids which are often used in or adjacent to gas turbine engines such as lubricating oils, kerosene and petroleum fuels and various hydraulic fluids, and the coating was found to be resistant to all these fluids.

Dry heating tests were carried out on the panels and it was found that the coating could give satisfactory protection up to some 550C.

EXAMPLE 2 Further tests were carried out on panels whose coating had been produced using the same mixtures but a different heating cycle. In this case the panels were electrophoretically coated, stoved for 30 minutes at 150C and then heated to 350C to drive off the acrylic resin. The porous coating was then impregnated and heat treated as before. We found equally satisfactory results with these test panels.

EXAMPLE 3 Further test panels were coated using a mixture comprising 31 grams of Synolac 84l/S as described above, 41.6 grams of the 400 grade aluminum powder and 27.3 grams of distilled water. The test pieces were coated by electrophoresis (or electro-printing) using an electrical current in the range of 5 to milliamps per square inch.

These test panels were then stoved and further coated as described in the previous examples, and were found to give satisfactory results in similar tests. It was additionally found that the mixture itself was less liable to settling out of the solid constituents than the previous mixture.

It should be noted that the process of electrophoresis or electro-printing as it is also known, may be carried out using a contrast current or a contrast voltage supply; in the latter case the current will vary over the coating process. In any case, the parameters of current and voltage which give the best results for a particular mixture are best determined by experiment, but we find that the current used will normally fall within the range 5 to 100 milliamps per square inch of surface to be cov ered.

In our experiments we have found that the coating should be such as to be electrically conductive so as to give galvanic resistance to corrosion. It is possible to arrange that the coating is conductive by the addition of a final heat treatment step in which the panel involved is heated to 560C for 2 hours, as was seen in the following example.

EXAMPLE 4 Test panels were made and coated in exactly the same manner as Example 1, but in this case after the final stoving the panels were further heat treated to 560C in air for 2 hours. The panels were tested and it was found that the coatings were electrically conductive and gave even better protection than the panels of Example 1.

It should be appreciated that this further heat treatment is entirely optional in that satisfactory coatings can be produced without this further step, although the coatings are improved by carrying it out.

Again, in the examples above the impregnation with potassium silicate was carried out by applying the solution by brushing. Clearly this could be equally well done by spraying or dipping.

It will be appreciated that the examples described above produce a single layer coating whose thickness is therefore limited to some 0.8 of a thousandth of an inch. We have found that further coatings can be applied by electrophoretic coating as above and burning out the acrylic resin followed by a second electrophoretic coating and burning out and subsequent impregnation of both porous layers. It would of course be possible to continue this process to give three or more layers.

It will be appreciated that the constituents of the mixtures involved could be varied within the limit explained above, however we believe that this method of coating is only applicable to steel components which are not necessarily of the composition exemplified above.

We claim:

1. A method of applying a coating to a steel part comprising the steps of firstly electrophoretically applying a coating of a mixture of aluminum powder and organic resin to the part, secondly heating the part to drive off the organic resin, and thirdly impregnating the coating with a high temperature resistant base material.

2. A method as claimed in claim 1 and in which the high temperature resistant base material is potassium silicate.

3. A method as claimed in claim 2 and comprising a further step in which the impregnated coating is stoved.

4. A method as claimed in claim 3 and comprising a still further step in which the coating is heated to increase its electrical conductivity.

5. A method as claimed in claim 4 and in which said subsequent heating comprises heating in air to 560C for 2 hours.

6. A method as claimed in claim 1 and in which the organic resin comprises an acrylic resin.

7. A method as claimed in claim 1 and in which the electrophoretic coating is performed from an aqueous solution.

8. A method as claimed in claim 6 and in which the solution comprises from one to two parts by weight of an acrylic resin, from 2 to 20 parts by weight of fine powdered aluminum, and water to make the required consistency.

9. A method as claimed in claim 7 and in which the electrophoretic coating is effected using a current density in the range 5 to milliamps per square inch.

10. A method as claimed in claim 1 and in which the high temperature resistant base material is applied as an aqueous solution comprising one part by volume of potassium silicate and two parts by volume of water.

11. A method as claimed in claim 1 and in which a plurality of said electrophoretic coatings are applied and the resin burnt out, the multiple coating then being impregnated with the base material.

12. A method as claimed in claim 1 and comprising burning the resin out of the electrophoretically deposited coating by heating to a temperature of 560C.

13. A method as claimed in claim 1 and in which the coating is finally stoved by heating to l50C.

14. A method of applying a high temperature and corrosion resistant coating to a steel part comprising the steps of:

a. electrophoretically applying to the steel part a coating of a mixture of a major portion of aluminum powder and a minor portion of an acrylic resin;

b. heating the thus-coated part to drive off the acrylic resin and diffuse the aluminum coating onto the steel part;

c. cooling the thus-heated part; and

d. impregnating the aluminum coating formed in steps (a) and (b) with the high temperature resistant base material potassium silicate. 

2. A method as claimed in claim 1 and in which the high temperature resistant base material is potassium silicate.
 3. A method as claimed in claim 2 and comprising a further step in which the impregnated coating is stoved.
 4. A method as claimed in claim 3 and comprising a still further step in which the coating is heated to increase its electrical conductivity.
 5. A method as claimed in claim 4 and in which said subsequent heating comprises heating in air to 560*C for 2 hours.
 6. A method as claimed in claim 1 and in which the organic resin comprises an acrylic resin.
 7. A method as claimed in claim 1 and in which the electrophoretic coating is performed from an aqueous solution.
 8. A method as claimed in claim 6 and in which the solution comprises from one to two parts by weight of an acrylic resin, from 2 to 20 parts by weight of fine powdered aluminum, and water to make the required consistency.
 9. A method as claimed in claim 7 and in which the electrophoretic coating is effected using a current density in the range 5 to 100 milliamps per square inch.
 10. A method as claimed in claim 1 and in which the high temperature resistant base material is applied as an aqueous solution comprising one part by volume of potassium silicate and two parts by volume of water.
 11. A method as claimed in claim 1 and in which a plurality of said electrophoretic coatings are applied and the resin burnt out, the multiple coating then being impregnated with the base material.
 12. A method as claimed in claim 1 and comprising burning the resin out of the electrophoretically deposited coating by heating to a temperature of 560*C.
 13. A method as claimed in claim 1 and in which the coating is finally stoved by heating to 150*C.
 14. A method of applying a high temperature and corrosion resistant coating to a steel part comprising the steps of: a. electrophoretically applying to the steel part a coating of a mixture of a major portion of aluminum powder and a minor portion of an acrylic resin; b. heating the thus-coated part to drive off the acrylic resin and diffuse the aluminum coating onto the steel part; c. cooling the thus-heated part; and d. impregnating the aluminum coating formed in steps (a) and (b) with the high temperature resistant base material potassium silicate. 